1. Field of the Invention
This invention relates to a punching apparatus for making or forming holes, and more particularly, to a punching apparatus for making holes in a workpiece or matter to be worked such as ceramic green sheet (referred to simply as "green sheet", hereinafter).
2. Background Art
It is generally known to use a multilayer printed-circuit board of ceramic as a substrate of a semiconductor integrated-circuit device, for example, used in a large-scale computer and the like. This kind of ceramic substrate or board is generally manufactured in such a manner that a basic substrate called green sheet, which is formed by a sheet-like sinter of alumina, is first formed therein with through holes by a punching apparatus and, then, a conductive paste of molybdenum (Mo) or tungsten (W) is printed on the surface of the substrate in the fixed pattern and, at the same time, filled in the inner walls of the through holes, and thereafter, a fixed number of these substrates are laminated and subjected to the sintering at a time.
By the way, the performance of the multilayer printed-circuit board or substrate depends on whether or not the through holes serving to electrically connect the printed circuit conductor layers of the multilayer printed-circuit ceramic substrate to each other are formed in correct positions with correct size and whether or not the through holes are formed without damaging the substrate around the through holes. For this reason, in case of manufacturing the multilayer printed-circuit ceramic substrate, it is an important manufacturing process that the through holes are formed in correct positions without damaging the substrate around the through holes.
Examples of the conventional punching apparatus for making through hole include the punching apparatus disclosed in Japanese Patent Unexamined Publication Nos. 1-171800 and 2-59299. This kind of punching apparatus comprises a lower die block having a flat upper surface on which a green sheet as a sheet-like matter to be worked is to be put and a plurality of first penetrating holes each extending from the upper surface perpendicularly thereto and located in a position where a hole is to be made in the matter to be worked, an upper die block having a lower surface arranged close to the upper surface of the lower die block so as to be opposed to the upper surface thereof and a plurality of second penetrating holes each extending from the lower surface perpendicularly thereto and located in a position opposite to an opening at the upper end of the first penetrating hole, and a plurality of punch pins each having a shaft portion and a cutting edge portion located at the lower end of the shaft portion and inserted in the second penetrating hole movably along the second penetrating hole so as to project the cutting edge portion out from the second penetrating hole of the upper die block, in which punching apparatus the upper die block has a bearing portion in the axial central part of each second penetrating hole for serving to support the shaft portion of the punch pin for free sliding movement along the second penetrating hole.
The present inventors use a punching apparatus having a working section of the construction shown in FIG. 10 in one of the inventors' companies, which apparatus was obtained by improving the punching apparatus disclosed in the above publications and, hence, is not known in public.
In the punching apparatus of FIG. 10, a green sheet 201 to be subjected to punching is put on a die base 202 of a lower die assembly 250 of the punching apparatus, and a die 203 is fixed in a hole 251 of the . die base 202.
An upper die body 260 of the punching apparatus comprises a base plate 204 as an upper plate, a punch pin support body 205 fixed to the underside of the base plate 204, and a stripper plate 206 as a lower plate fixed to the underside of the punch pin support body 205.
A sleeve 207 is inserted in a penetrating hole 261 of the punch pin support body 205, and a ball guide 208 is inserted in the sleeve 207.
In a penetrating hole 262 of the base plate 204, a lower spring shoe 209 is inserted so as to be brought into contact with the upper surface of the punch pin support body 205 and, further, a spring 210 is inserted so as to be brought into contact with the upper surface of the spring shoe 209.
Further, in the penetrating hole 262 of the base plate 204, an upper spring shoe 211 is inserted so as to be brought into contact with the spring 210. A pin holder 213 formed with a flange 212 serving to come in contact with the spring shoe 211 is inserted in the ball guide 208 so as to be movable in vertical directions A and B.
A pin sleeve 214 having a flange 214a is put on and inserted in the upper portion of the pin holder 213. A punch pin 215 having a cutting edge 216 formed at one end thereof and extending through the base plate 204, the punch pin support body 205 and the stripper plate 206 is inserted in the pin sleeve 214.
The pin sleeve 214 is fixed to the punch pin 215 by means of a screw 219 screwed to one end of the pin sleeve 214 with its one end 217 being in contact with a large diameter upper end 218 of the punch pin 215. Upward movement of the pin holder 213 in the direction A is limited by a stopper plate 220 fixed to the base plate 204.
A set bush 221 is disposed in and fixed to the pin holder 213 at a lower portion thereof for serving to set a horizontal position of a shaft portion 263 of the punch pin 215 adjacent to the cutting edge 216.
In a working section 200 of the above construction, as the green sheet 201 is put on the die base 202 in position, a driving means which is not shown drives the punch pin 215 to move downward in the axial direction B under the guidance of the ball guide 208, thereby making a through hole 222 in the green sheet 201.
The diameter of the through hole 222 to be formed in this way has been reduced with the increase in mounting density and degree of integration, and now, it is required to make a through hole of diameter less than 0.2 mm. Further, increase of the number of through holes 222 causes the diameter of the cutting edge 216 of the punch pin 215 to be reduced as well, and a clearance C between the cutting edge 216 of the punch pin 215 and the die 203 has been required to be about 0.01 to 0.03 mm, more preferably as small as 0.001 mm, on one side.
In addition, in order to improve the efficiency of the punching operation, it is also required to increase the number of punch pins 215 to be disposed in the punching apparatus. For this reason, the punch pin 215 to be built in the punching apparatus 200 is needed to become further thin, and now, the outside diameter of the shaft portion thereof has become about 0.3 to 1 mm.
However, the above punching apparatus 200 according to the proposed technique is constructed with the intention of positioning the punch pin 215 with high accuracy so that the positioning structure thereof is very complicated. In consequence, the number of component parts is increased, resulting in a problem that it is difficult to obtain a high positional accuracy of the punch pin 215 under the influence of not only the error in making the penetrating hole 261 in the punch pin support body 205 but also the error in finishing the parts to be disposed in the hole 261 including the sleeve 207, the ball guide 208 and the pin holder 213 as well as the cumulative error appearing when the parts are combined.
Further, since it is necessary to provide a structure for stopping the leakage of the lubricating oil from the ball guide 208, such as a groove 223, in the stripper plate 206, the set bush 221 of the punch pin 215 is disposed at a distance from the upper surface 224 of the die base 202 and the cutting edge 216 of the punch pin 215, resulting in a disadvantage that it is very difficult to obtain a high positional accuracy of the cutting edge 216 of the punch pin 215.
On the other hand, complication of the above structure and increase of the number of parts make it difficult to dispose a large number of punch pins 215 in the upper die block at high density.